Projection systems require a high quality light source. While projection systems have traditionally used discharge lamps as a light source, there is now interest in alternative light sources such as lasers. Lasers have several advantageous properties. They emit a high intensity light beam and have a very long operating lifetime. Laser light is collimated and has a narrow spectrum.
Even if laser sources have a very low étendue, when building a very high brightness projector from several individual laser modules each has a certain tolerance on the beam angles, and it becomes challenging to combine the light from different laser modules in the available &endue from a projection system. Adjustable stages can be used to mitigate some of the beam tolerances but add significant expense and alignment effort. Since lasers produce polarized light, two laser beams with orthogonal polarization can be combined without increasing the étendue by using a polarizing beam splitter (PBS).
For example, US 2009/0141242 A1 describes a digital image projector comprising a first polarized light source, a second polarized light source that is orthogonal in polarization state to the first polarized light source, a PBS disposed to direct light of either the first or the second polarization along a common illumination axis, a MEMS spatial light modulator, and projection optics for delivering imaging light from the MEMS spatial light modulator. Improved étendue matching between illumination and modulation components is achieved.
Such a combination of two laser beams with orthogonal polarization, in which the light flux for both polarization directions is balanced, is also advantageous if a Digital Light Processing (DLP) projector is used in combination with an external polarization system. For such application, it is important that either the light from the projector is fully polarized or that both polarization directions are represented equally. Without special measures, the original polarization from the laser source will be disturbed, in a manner that is different from one primary color with respect to another and that is non-uniform across the image. As a result the three-dimensional (3D) image produced with an external polarization system will suffer from discoloration and severe non-uniformities. Even worse, if moving components that suffer from birefringence are included in the optical path (such as a moving diffuser), the discoloration will vary over time.
Several solutions are known in the art to combine light rays with orthogonal polarization. Most PBS combiners use the principle that at a well defined incident angle (known as the Brewster angle) only s-polarized light is reflected, and apply a number of dielectric coating layers. PBS cubes typically show a very high reflection for s-polarized light (>99.5%), but the transmission for p-polarized light is rather low (about 90%-95%). Wire grid polarizers typically have efficiencies below 90% in both reflection and transmission. Further, in wire grid polarizers some of the energy is absorbed, which may lead to damaging the component at high illumination levels.
The majority of known polarizing beam splitters are generally designed for operation in broad ranges of wavelengths. However, the use of such broadband dichroic PBS results in relatively high light losses when broad spectral signals are transmitted while light in narrow wavelength bands is reflected. Furthermore, a broadband PBS can not reach efficiencies higher than 90 to 95% for angles different from the Brewster angle. This makes it difficult to combine two orthogonally polarized broadband light sources in an efficient way.
Efficiency is very important in designing a high brightness laser projector. Laser light is expensive and, therefore, a lower efficiency represents an important cost increase. With the currently known devices and methods for combining multiple laser beams of different polarization this can not be achieved.